An optical pickup device that is designed to be able to read optical recording media of different numerical aperture specifications, such as CD and DVD, by adding a ring belt to an objective lens and thereby apparently causing the light falling on the outer edges to vanish for a given wavelength by the interference action of the ring (Patent Document 1), is known. However, since it is not easy to form a ring belt for causing interference to a particular wavelength on the objective lens, this prior art design has had the problem of increased cost and reduced yield. A further problem has been that it is difficult to provide for a plurality of light wavelengths because the wavelength for which the interference action can be effectively caused is limited to a particular wavelength.
It is also known to provide a device which, using a single pickup, can detect information pits not only on a low-density disk but also on a high-density disk by selectively changing the polarization direction of light passing through a designated region of a liquid crystal filter and by eliminating, using a polarization beam splitter, the light whose polarization direction has been changed (or not changed) (Patent Document 2). To eliminate the unwanted light, the polarization beam splitter must invariably be inserted in the light path, but the problem has been that, because of the provision of the polarization beam splitter, the amount of light decreases or the freedom of design of the light path is limited. There has also been the problem that the polarization beam splitter is expensive, correspondingly increasing the cost of the device.
There is known a device in which a voltage is applied to a designated region of a liquid crystal panel, thereby causing the designated region to act as a λ/4 plate and allowing only the light passed through that region to be directed to a light detector by a polarization beam splitter (Patent Document 3). With this device, the diameter of the light beam that passes through the liquid crystal panel can be varied by selectively varying the region to be caused to act as the λ/4 plate. This is equivalent to varying the numerical aperture of the objective lens and, thus, this single device can be used for both a CD and a DVD. However, since the refractive index of the liquid crystal changes with temperature, there has been the problem that a strict temperature control mechanism becomes necessary in order to accurately operate the liquid crystal panel as a λ/4 panel and, as a result, the complexity of the structure increases correspondingly. Furthermore, as the refractive index of the liquid crystal changes with wavelength, there has also been the problem that light of a plurality of wavelengths cannot be used. A further problem has been that light that has not been accurately rotated through λ/4 is directed back to the light detector as noise.
There is also known a device in which wavelength-selective diffraction gratings arranged at equally spaced intervals are inserted in a light path in such manner that light of a first wavelength is allowed to pass freely through the wavelength-selective diffraction gratings, while light of a second wavelength is diffracted to outside the optical axis by the wavelength-selective diffraction gratings (Patent Document 4). With this device, by using the first wavelength for a DVD and the second wavelength for a CD, both CD and DVD readout can be accomplished using a single objective lens. However, as the wavelength-selective diffraction gratings arranged at precisely equally spaced intervals are not easy to fabricate because of their geometry, this prior art device has had the problem of increased cost and reduced yield. Furthermore, the wavelength that can be effectively diffracted is limited to a particular wavelength, but this wavelength varies with the temperature of the light source; therefore, there has been the problem that a strict temperature control mechanism becomes necessary in order to effectively achieve diffraction and, as a result, the complexity of the structure is correspondingly increased.
Patent Document 1: JP-A-2003-344759 (FIG. 1)
Patent Document 2: JP-B-3048768 (FIG. 1)
Patent Document 3: JP-B-3476989 (FIGS. 1 and 3)
Patent Document 4: JP-Y-3036314 (FIG. 3)